Consider a packed bed of solid aluminum spheres (75 mm diameter, ρ= 2700 kg/m3, c= 950 J/kg-K, k= 240 W/m-K) that is charged (hot gas at Tgi= 300o C flows through the system). If the initial temperature of the spheres is Ti= 25oC and the convection coefficient h= 75 W/m2-K, how long does it take a sphere near the inlet of the system to accumulate 90 % of the maximum possible thermal energy ?